The present invention relates to a piston type compressor. More particularly, the present invention pertains to a compressor that decreases pressure loss at the last stage of piston discharge strokes.
Japanese Unexamined Patent Publications Nos. 8-261150 and 10-68382 disclose piston type compressors.
FIG. 11 illustrates part of the piston type compressor of the publications. A piston 81 is reciprocally housed in a cylinder bore 82. A valve plate 95 separates the cylinder bore 82 from a suction chamber 83 and from a discharge chamber 84. The valve plate 95 includes a main plate 85, a first sub plate 89 and a second sub plate 91. The first and second sub plates 89, 91 sandwich the main plate 85. A suction port 86 and a discharge port 87 are formed in the valve plate 95. The first sub plate 89 includes a suction valve flap 88. The suction valve flap 88 corresponds to the suction port 86. The second sub plate 91 has a discharge valve flap 90. The discharge valve flap 90 corresponds to the discharge port 87.
A compression chamber 92 is defined by the end face of the piston 81 and the first sub plate 89 in the cylinder bore 82. When the piston 81 is moved from the top dead center position to the bottom dead center position, that is, when the piston 81 is in the suction stroke, refrigerant gas in the suction chamber 83 is drawn into the compression chamber 92 through the suction port 86 and the suction valve flap 88. When the piston 81 moves from the bottom dead center position toward the top dead center position, that is, when the piston 81 is in the discharge stroke, the gas in the compression chamber 92 is compressed to a predetermined pressure. The gas is then discharged to the discharge chamber 84 through the discharge port 87 and the valve flap 90.
As shown in FIG. 12, the ports 86 and 87 are located radially inside of the wall of the cylinder bore 82.
When the piston 81 is at the last stage of the discharge stroke, that is, when the piston 81 is in the vicinity of the top dead center position, gas in the compression chamber 92 flows to the discharge port 87 through a narrow space between the end of the piston 81 and the first sub plate 89. This causes a pressure loss. The pressure loss decreases the compression efficiency of the compressor.
Compressors that are used in vehicle air conditioners typically use fluorocarbon as refrigerant. However, the recent trend is to replace fluorocarbon by carbon dioxide to decrease the influence of the refrigerant on the environment.
Carbon dioxide refrigerant requires a higher compression rate (for example, ten times higher) than fluorocarbon refrigerant. Thus, the pressure loss mentioned above is much more significant in compressors using carbon dioxide as a refrigerant.